As the four heme groups of the hemoglobin molecule become progressively saturated with ligands, such as oxygen or carbon monoxide, the ligand affinity of the unreacted heme groups increases. This necessitates a type of communication between these heme groups. It is the objective of this investigation to understand the molecular basis for this and the other related allosteric phenomena in hemoglobin. The functional properties of the different structural states of the molecule will be determined and the sequence of structural changes that accompany the saturation of the molecule with ligand explored. The dependence of the molecular behavior on the reaction of the two dissimilar subunits to form the intact molecule will be assessed by comparing the properties of the isolated subunits to those of the complete molecule in various states of ligand saturation. The properties of antibody populations that react with single sites on heme protein antigens will be explored and the equilibrium and kinetic constants for their reactions with their specific antigens determined under a variety of conditions. Such site specific antibodies will be used as probes of structural changes at the surface of the hemoglobin molecule.